1. Field of the Invention
The present invention relates to a light emitting screen structure (light emitting substrate) for constituting an image forming apparatus such as an image display apparatus in combination with an electron emitting device, and to an image forming apparatus utilizing such light emitting substrate.
2. Related Background Art
An electron emitting apparatus utilizing an electron emitting device has been applied for example to an image forming apparatus. For example, there is known a flat type electron beam display panel, formed by arranging an electron source substrate provided with a plurality of cold cathode electron emitting devices, and an anode substrate provided with a metal back or a transparent electrode for accelerating electrons emitted from the electron emitting devices, and a phosphor in a mutually opposed relationship and by evacuating a gap between such substrates. The flat type electron beam display panel can achieve a lighter weight and a larger image size in comparison with the currently popular cathode ray tube (CRT). Also it can provide a higher luminance and a higher quality in the image, in comparison with other flat display panels such as a liquid crystal flat display panel, a plasma display panel or an electroluminescent display.
In such image forming apparatus of the type in which a high voltage is applied between the cold cathode multi electron source and the aforementioned metal back or transparent electrode for accelerating the electrons, a higher voltage application is advantageous for maximizing the luminance of the emitted light. Also, in certain device types, the emitted electron beam diverges before reaching the counter electrode, so that a shorter distance between the electrodes is preferable for realizing a display of a high resolution.
In such configuration, however, as a high electric field is inevitably formed between the opposed electrodes, a discharge may be generated to destruct the electron emitting device. Also in case of such discharge, a current is generated in a concentrated manner thereby causing a light spot phenomenon in a part of the displayed image.
For avoiding such drawbacks, it is required to reduce the frequency of discharge or to render the destruction by discharge less possible.
The destruction by discharge is assumed to be caused by a large current concentrated in a single position within a short time resulting in a heat generation which destructs the electron emitting device, or by an instantaneous increase in the voltage on the electron emitting device, leading to the destruction of the electron emitting device.
For decreasing the current causing the destruction by discharge, there is conceived a method of serially limiting a limiting resistor between the anode electrode and the power source, as shown in FIG. 7. However, in case of connecting devices of 500 units in the vertical direction by 1000 units in the horizontal direction with row and column wirings and driving these elements in a line-sequential mode, about 1000 devices are simultaneously turned on and such method leads to following drawbacks.
As about 1000 devices are simultaneously turned on with an emission current assumed as 5 μA per device, there results a current of 0 to 5 mA flowing into the anode. In case of externally inserting a serial resistance of 1 MΩ to the anode as shown in FIG. 7 and applying a voltage of 10 kV to the anode, there results a voltage drop of 0 to 5 kV depending on the number of devices turned on simultaneously. As a result, there will result an unevenness in the luminance of 50% at maximum.
Also as a high voltage is applied to the opposed flat plates (face plate and rear plate) 71, 72, it is necessary to also consider a charge accumulated as a capacitor. For example, in case the cathode and the anode shown in FIG. 7 have an area of 100 cm2 and a distance of 1 mm with a potential difference of 10 kV, the accumulated charge reaches 1×10−6 coulomb which causes a current concentration of 1 A in one position even when it is discharged in 1 μsec. Such discharge current leads to the destruction of the devices, so that the configuration of FIG. 7 cannot provide a sufficient solution to the drawback even if the aforementioned unevenness in luminance is absent.
For these drawbacks, the present applicant proposed, in Patent Reference 1, to divide an electrode for voltage application in a non-parallel manner to the direction of a scanning wiring and to provide a resistor between the electrode and accelerating voltage application means, thereby suppressing a discharge current generated between the mutually opposed flat plates.
FIG. 8 shows an example thereof and FIG. 9 shows an equivalent circuit thereof. In the drawings, there are shown a divided electrode 81, a resistor 82, a high voltage terminal 83, a high resistance area 84, a common electrode 85, a face plate 91 and a rear plate 92. Each divided electrode 81 (for example formed by an ITO film) is connected at an end thereof with the common electrode 85 through the resistor 82 (for example formed by an NiO film) whereby a high voltage can be applied from the terminal 83. In this configuration, the electrode at the side of the face plate 91 shown in FIG. 9 is divided, and a high resistor R1 is inserted in each divided electrode to reduce the capacity of the capacitor thereby reducing a discharge current Ib2. It is thus made possible to reduce an increase in the device voltage by a discharge current and to alleviate the damage at the discharge.
Also Patent Reference 2 discloses a cold cathode field emission display apparatus satisfying a relation Va/Lg<1 (kV/μm) between an anode voltage Va and a gap Lg between the anode electrode units. It is proposed, by such configuration, to suppress a discharge among the anode electrode units at an abnormal discharge, thereby suppressing the magnitude of the discharge.
Patent Reference 1: Japanese Patent No. 3199682 (EP866491A)
Patent Reference 2: Japanese Patent Application Laid-open No. 2004-47408
As explained in the foregoing, in an image forming apparatus constituted with electron emitting devices, there is desired a further reduction of the discharge current in the light emitting substrate (anode substrate), in order to reduce the damage to the electron emitting device in case of an abnormal discharge. It is particularly desired, in case of an abnormal discharge between the anode and the cathode, to suppress a discharge secondarily generated between adjacent anode electrodes. On the other hand, it is desired to reduce a gap between the adjacent anode electrodes in order to obtain an image of a higher definition.